The present invention pertains to the electronic communication art and, in particular, to an improved tone detector for use therein.
Tone detector systems have found extensive application, especially in the two-way communication art. There, for example, a dispatcher located at a central base station may wish to selectively communicate with one or more of several mobile units. Each mobile unit is assigned a particular tone code such that when the transmitted message from the dispatcher contains the appropriate subaudible tone the mobile realizes it is being addressed and processes the remainder of the message.
The heretofore dominant method of tone detecting has employed mechanical reeds which are tuned to the tones to be detected. The reeds have a high "Q" whereby when a received signal contains the resonant frequency the reed oscillates thereby notifying detector circuitry that the tone is present.
A particular problem with the reed type tone detectors has been that, due to its high Q the reed is slow to respond to a received tone and, once the tone ceases, the reed continues oscillating for a significant time interval. Thus, the total system throughput time is degraded by the reed delay times.
A particularly effective way to minimize reed turnoff time is described in U.S. Pat. No. 2,974,221 issued Mar. 7, 1961, invented by Robert Peth, and assigned to the same assignee of the present invention. There, each transmitting station includes circuitry which, upon the end of a transmitted message generates a reverse tone burst which, when received at the receiving station, significantly reduces the reed delay turnoff time. While the Peth invention has significantly improved reed type tone detectors, there has remained a long felt need for a tone detector system which minimizes both turn on and turn off detect times and does not require reverse tone burst generating circuitry.